The desulfurization of flue gas, particularly flue gas from power plants, has been the subject of considerable study. Air quality laws, both at the federal and state level, have set increasingly stringent emissions standards, especially for such known pollutants as sulfur oxides. Because coal and oil-fired electric power generating plants can discharge large quantities of sulfur oxides as combustion by-products, much effort has focused on the desulfurization of flue gas to reduce power plant sulfur dioxide emissions to permissible levels.
Substantial effort has been expended to develop a flue gas desulfurization process which removes substantially all of the sulfur dioxide from the flue gas. Although existing desulfurization process chemistries have achieved high sulfur dioxide removal, these processes have also created other environmental problems. One particularly notable environmental problem to be solved is the disposal of the large quantities of solids produced by the most widely used flue gas desulfurization processes. In the past, flue gas desulfurization byproduct solids have been dumped in landfills. However, available landfill space is rapidly diminishing and will not be able to accommodate the disposal of flue gas desulfurization process waste solids if they continue to be generated at the current rate.
One of the available processes for removing sulfur dioxide from flue gas has a high effective sulfur dioxide removal rate. This process, variations of which are disclosed in U.S. Pat. Nos. 3,914,378; 3,919,393 and 3,319,394 to Selmeczi, is a wet scrubbing process which uses magnesium and lime to precipitate sulfur oxides in the flue gas as calcium sulfite and calcium sulfate solids. A slurry is used as the scrubbing medium, and lime is added directly to the scrubbing slurry to precipitate calcium salts. The calcium sulfates and sulfites are precipitated in the presence of soluble magnesium. These solids form a sludge which retains relatively large amounts of water and thus is difficult to dispose of. Consequently, this process is a costly way to desulfurize flue gas.
Although the magnesium-enhanced lime flue gas desulfurization process disclosed in the aforementioned patents effectively removes sulfur dioxide from flue gas, the poor quality of the calcium sulfite solids produced has been a major drawback to the use of this process. A significant aspect of the magnesium-enhanced lime scrubbing process is that magnesium must be kept at 3000-8000 ppm in the liquid phase to achieve the high SO.sub.2 removals desired. The soluble magnesium carries with it high concentrations of sulfites and sulfates. The magnesium and sulfates act as crystal habit modifiers, and the calcium sulfite crystals formed are relatively small, needlelike and fragile. As a result, the attrition of these fragile crystals that occurs as they are recirculated through the absorber produces fine solids that dewater at low rates and retain more water than the solids generated by lime or limestone flue gas desulfurization processes. Large thickeners and filters are required to dewater the solids, and the waste disposal problem is costly. In addition, the use of a slurry as the scrubbing medium produces deposits of solids in the quiescent areas of the scrubbing system. The slurry solids also cause abrasion on the pumps, piping nozzles and other system components.
The prior art has proposed a system for removing solids in a wet lime or limestone flue gas desulfurization system which changes crystal size and habit by feeding spent scrubbing liquor to agitated and quiescent crystallization zones in the presence of a crystal habit modifier in U.S. Pat. No. 4,294,807 to Randolph. The system described by Randolph does improve solids dewatering when lime is used for scrubbing. However, this system employs a very different chemistry from a magnesium-enhanced lime flue gas desulfurization process and will not necessarily solve the solids dewatering problem. For example, additives designed to enhance sulfur dioxide removal or control scale in a lime or limestone flue gas desulfurization process are lost by coprecipitation with the product solids in a magnesium-enhanced lime flue gas desulfurization process. There is no suggestion in this patent, moreover, that the crystallization system described therein would improve solids quality or dewatering in any flue gas desulfurization process other than one based on lime or limestone.
The prior art, therefore, has failed to provide a wet magnesium-enhanced lime flue gas desulfurization system or process that effectively improves both the properties and the dewatering characteristics of the solids produced by this process. Consequently, there is a need for a magnesium-enhanced lime flue gas desulfurization system and process which produce both a high sulfur dioxide removal efficiency and easily dewatered and disposed of waste solids.